Class Parameter

Lib/inspect.py:2654–2816

Represents a parameter in a function signature. Has the following public attributes: * name : str The name of the parameter as a string. * default : object The default value for the parameter if specified. If the parameter has no default value, this attribute i

class Parameter:
    """Represents a parameter in a function signature.

    Has the following public attributes:

    * name : str
        The name of the parameter as a string.
    * default : object
        The default value for the parameter if specified.  If the
        parameter has no default value, this attribute is set to
        `Parameter.empty`.
    * annotation
        The annotation for the parameter if specified.  If the
        parameter has no annotation, this attribute is set to
        `Parameter.empty`.
    * kind
        Describes how argument values are bound to the parameter.
        Possible values: `Parameter.POSITIONAL_ONLY`,
        `Parameter.POSITIONAL_OR_KEYWORD`, `Parameter.VAR_POSITIONAL`,
        `Parameter.KEYWORD_ONLY`, `Parameter.VAR_KEYWORD`.
        Every value has a `description` attribute describing meaning.
    """

    __slots__ = ('_name', '_kind', '_default', '_annotation')

    POSITIONAL_ONLY         = _POSITIONAL_ONLY
    POSITIONAL_OR_KEYWORD   = _POSITIONAL_OR_KEYWORD
    VAR_POSITIONAL          = _VAR_POSITIONAL
    KEYWORD_ONLY            = _KEYWORD_ONLY
    VAR_KEYWORD             = _VAR_KEYWORD

    empty = _empty

    def __init__(self, name, kind, *, default=_empty, annotation=_empty):
        try:
            self._kind = _ParameterKind(kind)
        except ValueError:
            raise ValueError(f'value {kind!r} is not a valid Parameter.kind')
        if default is not _empty:
            if self._kind in (_VAR_POSITIONAL, _VAR_KEYWORD):
                msg = '{} parameters cannot have default values'
                msg = msg.format(self._kind.description)
                raise ValueError(msg)
        self._default = default
        self._annotation = annotation

        if name is _empty:
            raise ValueError('name is a required attribute for Parameter')

        if not isinstance(name, str):
            msg = 'name must be a str, not a {}'.format(type(name).__name__)
            raise TypeError(msg)

        if name[0] == '.' and name[1:].isdigit():
            # These are implicit arguments generated by comprehensions. In
            # order to provide a friendlier interface to users, we recast
            # their name as "implicitN" and treat them as positional-only.
            # See issue 19611.